The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. (See, Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429; Kunz et al., Cell 1993, 73, 585-596; Garcia-Bustos et al., EMBO J. 1994, 13, 2352-2361).
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events as described above. These diseases include, but are not limited to, cancer and other proliferative disorders. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
The c-Met proto-oncogene encodes the Met receptor tyrosine kinase. The Met receptor is a 190 kDa glycosylated dimeric complex composed of a 50 kDa alpha chain disulfide-linked to a 145 kDa beta chain. The alpha chain is found extracellularly while the beta chain contains transmembrane and cytosolic domains. Met is synthesized as a precursor and is proteolytically cleaved to yield mature alpha and beta subunits. It displays structural similarities to semaphorins and plexins, a ligand-receptor family that is involved in cell-cell interaction. The ligand for Met is hepatocyte growth factor (HGF), a member of the scatter factor family and has some homology to plasminogen [Longati, P. et al., Curr. Drug Targets 2001, 2, 41-55); Trusolino, L. and Comoglio, P. Nature Rev. Cancer 2002, 2, 289-300].
Met functions in tumorigenesis and tumor metastasis. Chromosomal rearrangements forming Tpr-met fusions in an osteoclast cell line resulted in constitutively active Met receptors and transformation (Cooper, C. S. et al., Nature 1984, 311, 29-33). Met mutants exhibiting enhanced kinase activity have been identified in both hereditary and sporadic forms of papillary renal carcinoma (Schmidt, L. et al., Nat. Genet. 1997, 16, 68-73; Jeffers, M. et al., Proc. Nat. Acad. Sci. 1997, 94, 11445-11500). Expression of Met along with its ligand HGF is transforming, tumorigenic, and metastatic (Jeffers, M. et al., Oncogene 1996, 13, 853-856; Michieli, P. et al., Oncogene 1999, 18, 5221-5231). HGF/Met has been shown to inhibit anoikis, suspension-induced programmed cell death (apoptosis), in head and neck squamous cell carcinoma cells. Anoikis resistance or anchorage-independent survival is a hallmark of oncogenic transformation of epithelial cells (Zeng, Q. et al., J. Biol. Chem. 2002, 277, 25203-25208).
MET is overexpressed in a significant percentage of human cancers and is amplified during the transition between primary tumors and metastasis. To investigate whether this oncogene is directly responsible for the acquisition of the metastatic phenotype, Giordano et al. exploited a single-hit oncogenic version of MET that was able to transform and to confer invasive and metastatic properties to nontumorigenic cells, both in vitro and in nude mice. They found a point mutation in the signal transducer docking site of MET that increased the transforming ability of the oncogene, but abolished its metastatic potential. They concluded that the metastatic potential of the MET oncogene relies on the properties of its multifunctional docking site, and that a single point mutation affecting signal transduction can dissociate neoplastic transformation from metastasis. Giordano, S., et al, Proc. Nat. Acad. Sci. 94: 13868-13872, 1997.
c-Met is implicated in various cancers, especially renal cancer. It was found that the beta-subunit of the c-Met protooncogene product is the cell-surface receptor for hepatocyte growth factor. It was also identified that the hepatocyte growth factor receptor is the c-met proto-oncogene product. Bottaro, D. P., et al, Science 251: 802-804, 1991.
HGF/Met signaling is involved in cell adhesion and motility in normal cells and plays a major role in the invasive growth that is found in most tissues, including cartilage, bone, blood vessels, and neurons (reviewed in Comoglio, P. M. and Trusolino, L. J. Clin. Invest. 2002, 109, 857-862). Dysfunctional activation or increased numbers of Met is likely to contribute to the aberrant cell-cell interactions that lead to migration, proliferation, and survival of cells that is characteristic of tumor metastasis. Activation of Met induces and sustains a variety of tumors [Wang, R. et al., J. Cell. Biol. 2001, 153, 1023-1034; Liang, T. J. et al., J. Clin. Invest. 1996, 97, 2872-2877; Jeffers, M. et al., Proc. Nat. Acad. Sci. 1998, 95, 14417-14422] while loss of Met inhibits growth and invasiveness of tumor cells [Jiang, W. G. et al., Clin. Cancer Res. 2001, 7, 2555-2562; Abounader, R. et al., FASEB J. 2002 16, 108-110]. Increased expression of Met/HGF is seen in many metastatic tumors including colon (Fazekas, K. et al., Clin. Exp. Metastasis 2000, 18, 639-649), breast (Elliott, B. E. et al., 2002, Can. J. Physiol. Pharmacol. 80, 91-102), prostate (Knudsen, B. S. et al., Urology 2002, 60, 1113-1117), lung (Siegfried, J. M. et al., Ann. Thorac. Surg. 1998, 66, 1915-1918), and gastric (Amemiya, H. et al., Oncology 2002, 63, 286-296).
Further demonstration of the role Met plays in metastasis was shown by Giordano, et al. (2002) who presented evidence for cross-talk between the semaphorin 4D (SEMA4D; 601866) receptor, plexin B1 (PLXNB1; 601053), and MET during invasive growth in epithelial cells. Binding of SEMA4D to PLXNB1 stimulated tyrosine kinase activity of MET, resulting in tyrosine phosphorylation of both receptors. This effect was not found in cells lacking MET expression. Giordano, S., et al: The Semaphorin 4D receptor controls invasive growth by coupling with Met. Nature Cell Biol. 4: 720-724, 2002.
HGF-Met signaling has also been associated with increased risk of atherosclerosis (Yamamoto, Y. et al., J. Hypertens. 2001, 19, 1975-1979; Morishita, R. et al., Endocr. J. 2002, 49, 273-284) and increased fibrosis of the lung (Crestani, B. et al., Lab. Invest. 2002, 82, 1015-1022.
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of c-Met, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.